Vulnerability in Jupyter Server
CVE-2026-40934
Jupyter Server is the backend for Jupyter web applications. In versions 2.17.0 and earlier, the secret used to sign authentication cookies is persisted to a static file at ~/.local/share/jupyter/runtime/jupyter_cookie_secret and is never rotated when a user changes their password. After a password reset and server restart, any previously issued authentication cookie remains cryptographically valid because the signing key has not changed. An attacker who has captured a session cookie through any means retains full authenticated access to the server regardless of subsequent password changes. This affects deployments using password-based authentication, particularly shared or public-facing servers where credential rotation is expected to revoke existing sessions. This issue has been fixed in version 2.18.0.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.004 (30.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 31st percentile — 30.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Jupyter Server
- Jupyter-Server Jupyter Server — versions < 2.18.0
Weakness classification (CWE)
References
- github.com/jupyter-server/jupyter_server/security/advisories/GHSA-5mrq-x3x5-8v8f (Exploit, Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-40934?
- CVE-2026-40934 is a medium-severity vulnerability in Jupyter Server, classified under Insufficient Session Expiration. CVSS score: 6.8/10. Published 2026-05-05.
- How severe is CVE-2026-40934?
- Medium severity. CVSS v3 base score is 6.8 out of 10.